Known as the “Apostle of the English,” Augustine (also called Austin; not to be confused with Augustine of Hippo, who came earlier) founded the Christian Church in southern England and became the first archbishop of Canterbury.
Augustine was selected as leader of a mission of several dozen monks to England in the 590s. The mission was well-received by King Ethelbert of Kent, who offered them places to live, allowed them to preach, and supported their efforts of conversion. Legend says he baptized 10,000 souls on Christmas day in 598.
The introduction of Christianity required the organized observation of Saints’ days and other holy days, to which end Augustine introduced the Julian calendar as used by Rome since Julius Caesar had settled on it in 46BC.
The most significant of all Christian holy days, Christmas, was the date at which this introduction is said to have been formalized in Kent. The entirety of England would not adopt the Julian system until the Council of Chelsea in 816 AD.